Recombinant vaccines have been developed with the progress of recombinant DNA technology, allowing the modification of viral genomes to produce modified viruses. In this manner, it has been possible to introduce genetic sequences into non-pathogenic viruses, so that they encode immunogenic proteins to be expressed in target cells upon infection, in order to develop a specific immune response in their host.
Such vaccines constitute a major advance in vaccine technology (Kutzler et al., Nat Rev Genet, 9(10): 776-788, 2008). In particular, they have the advantage over traditional vaccines of avoiding live (attenuated) virus and eliminating risks associated with the manufacture of inactivated vaccines.
Gene delivery using modified retroviruses (retroviral vectors) was introduced in the early 1980s by Mann et al. (Cell, 33(1):153-9, 1983). The most commonly used oncogenic retroviral vectors are based on the Moloney murine leukemia virus (MLV). They have a simple genome from which the polyproteins Gag, Pol and Env are produced and are required in trans for viral replication (Breckpot et al., 2007, Gene Ther, 14(11):847-62; He et al. 2007, Expert Rev vaccines, 6(6):913-24). Sequences generally required in cis are the long terminal repeats (LTRs) and its vicinity: the inverted repeats (IR or att sites) required for integration, the packaging sequence ψ, the transport RNA-binding site (primer binding site, PBS), and some additional sequences involved in reverse transcription (the repeat R within the LTRs, and the polypurine tracts, PPT, necessary for plus strand initiation). To generate replication-defective retroviral vectors, the gag, pol, and env genes are generally entirely deleted and replaced with an expression cassette.
Retroviral vectors deriving from lentivirus genomes (i.e. lentiviral vectors) have emerged as promising tools for both gene therapy and immunotherapy purposes, because they exhibit several advantages over other viral systems. In particular, lentiviral vectors themselves are not toxic and, unlike other retroviruses, lentiviruses are capable of transducing non-dividing cells, in particular dendritic cells (He et al. 2007, Expert Rev vaccines, 6(6):913-24), allowing antigen presentation through the endogenous pathway.
Lentiviruses are linked by similarities in genetic composition, molecular mechanisms of replication and biological interactions with their hosts. They are best known as agents of slow disease syndromes that begin insidiously after prolonged periods of subclinical infection and progress slowly; thus, they are referred to as the “slow” viruses (Narayan et al., 1989, J Gen Virol, 70(7):1617-39). They have the same basic organization as all retroviruses but are more complex due to the presence of accessory genes (e.g., vif, vpr, vpu, nef, tat, and rev), which play key roles in lentiviral replication in vivo.
Lentiviruses represent a genus of slow viruses of the Retroviridae family, which includes the human immunodeficiency viruses (HIV), the simian immunodeficiency virus (Sly), the equine infectious encephalitis virus (EIAV), the caprine arthritis encephalitis virus (CAEV), the bovine immunodeficiency virus (BIV) and the feline immunodeficiency virus (FIV). Lentiviruses can persist indefinitely in their hosts and replicate continuously at variable rates during the course of the lifelong infection. Persistent replication of the viruses in their hosts depends on their ability to circumvent host defenses.
The design of recombinant integrative lentiviral vectors is based on the separation of the cis- and trans-acting sequences of the lentivirus. Efficient transduction in non-dividing cells requires the presence of two cis-acting sequences in the lentiviral genome, the central polypurine tract (cPPT) and the central termination sequence (CTS). These lead to the formation of a triple-stranded DNA structure called the central DNA “flap”, which maximizes the efficiency of gene import into the nuclei of non-dividing cells, including dendritic cells (DCs) (Zennou et al., 2000, Cell, 101(2) 173-85; Arhel et al., 2007, EMBO J, 26(12):3025-37).
Dendritic cells are of primary importance for antigen presentation because they constitute the main class of antigen presenting cells (APCs) whose primary function is to present antigens and initiate an immune response.
To generate an immune response, antigenic proteins must be processed by cells into peptides that are displayed on the cell surface by major histocompatibility complex proteins (MHCs). Circulating APCs present the peptide-MHC complexes to T cells in the draining lymph nodes, where they interact with T cell receptors, and, in conjunction with co-stimulatory signals, activate the T cells.
A variety of studies have shown that inoculation with lentiviral vectors leads to antigen presentation by DCs and strong activation of antigen specific cytotoxic T lymphocytes (CTLs; CD8+ T cells). Therefore, lentiviral vectors have been engineered for the last 10 years for gene transfer and immunotherapy applications.
Lentiviral vectors have been improved in their safety by removal of the LTR U3 sequence, resulting in “self-inactivating” vectors that are entirely devoid of viral promoter and enhancer sequences originally present within the LTRs.
The lentiviral particles, which contain lentiviral vectors, can be produced by recombinant technology upon transient transfection of HEK 293T human cultured cells by different DNA plasmids:
(i) a packaging plasmid, which expresses at least the Gag, Pol Rev, Tat and, in some cases, structural and enzymatic proteins necessary for the packaging of the transfer construct;
(ii) a transfer plasmid, containing an expression cassette and HIV cis-acting factors necessary for packaging, reverse transcription, and integration; and
(iii) an envelope-encoding plasmid, in most cases the glycoprotein of vesicular stomatitis virus (VSV.G), a protein that allows the formation of mixed particles (pseudotypes) that can target a wide variety of cells, especially major histocompatibility (MHC) antigen-presenting cells (APCs), including DCs.
This procedure allows obtaining transient production of lentiviral particle vectors by the transfected cells. However, the lentiviral particle vectors may also be continuously produced by cells by stably inserting the packaging genes, the proviral coding DNA, and the envelope gene into the cellular genome. This allows the continuous production of lentiviral particle vectors by the cells without the need for transient transfection. Of course, a combination of these procedures can be used, with some of the DNAs/plasmids integrated into the cellular genome and others provided by transient transfection.
Non-integrative lentiviral vectors are being designed to mitigate the risks of potential oncogenesis linked to insertional mutagenesis events, particularly for vaccination purposes:
In vaccination based on direct injection of antigen-encoding integrative lentiviral vectors, transduced cells expressing the relevant antigen become targets of the elicited immune response and are eliminated within a few days or weeks from the vaccinated organism.
In addition, deletion in the U3 region of the 3′ LTR of the viral promoter and enhancer sequences in self-inactivating lentiviral vectors limits the likelihood of endogenous promoter activation. This deletion directly addresses the experiences gained from the SCID-X1 gene therapy trial carried out in 1998-1999, performed with Moloney virus-based retroviral vectors on children suffering from a rare form of X-linked (SCID-X1 gene) severe immunodeficiency disease (Cavazzana-Calvo et al., 2000, Science., 288(5466):669-72). During this trial, four of nine children developed leukemia as a result of the integration of the Moloney-derived retroviral vector at close proximity to the human LM02 proto-oncogene (Hacein-Bey-Abina et al., 2008, J. Clin. Invest., 118(9):3132-3142). It was demonstrated that malignancy was the consequence of the proximity of the viral U3 promoter/enhancer to the LM02 proto-oncogene.
Enhancers are cis-acting sequences, which can act as transcriptional activators at a distance. They have been widely employed in viral derived vectors because they appear to be the most efficient for obtaining transgene strong expression in a variety of cell types, in particular DCs (Chinnasamy, Chinnasamy et al., 2000, Hum Gene Ther 11(13):1901-9; Rouas et al., 2008, Cancer Gene Ther 9(9):715-24; Kimura et al., 2007, Mol Ther 15(7):1390-9; Gruh et al., 2008, J Gene Med 10(1) 21-32). However, given the safety issue of insertional mutagenesis, such transcriptional enhancer sequences should be deleted from the lentiviral vector constructs to abolish the risk of insertional mutagenesis by enhancer proximity effect. This enhancer proximity effect is by far the most frequent mechanism of insertional mutagenesis and is the only effect described in human or animal cases of tumorigenic events after gene transfer.
Thus, there is a need to develop retroviral, particularly lentiviral vectors, which do not include viral enhancers and which allow sufficient expression of transgenes encoding immunogenic peptides, if possible as much expression as that observed when using the CMV promoter.
A recent study has reported on the replacement of viral promoters by DC-specific promoters deriving from major histocompatibility complex class II genes (MHC class II) (Kimura et al., 2007, Mol Ther 15(7):1390-9) and dectin-2 genes (Lopes et al., 2008, J Virol 82(1):86-95). The dectin-2 gene promoter used in Lopes et al. contains a putative enhancer and an adenoviral conserved sequence (inverted terminal repeats in adenovirus promoter) (Bonkabara et al., 2001, J. Immunology, 167:6893-6900). The MHC class II gene promoter used by Kimura et al. does not contain any known enhancer.
Yet, without an enhancer, the MHC class II promoter was found not to provide sufficient transgene expression in DCs. In particular, lentiviral vectors including MHC class II promoters did not provoke an immune reaction in immunocompetent C57BL/6 mice, in contrast to the immune responses observed with CMV promoters/enhancers. Although integration and persistent transgene expression were observed after injection in mice, the lentiviral vectors transcribed through MHC class II promoters failed to stimulate an antigen-specific CD8+ cytotoxic T-lymphocyte response, even after vaccination boost. The authors of these studies therefore concluded that the use of MHC class II promoters was of interest only for applications where persistence of expression is sought as in gene replacement therapy, but not in the context of immunotherapy.
Thus, the MHC class II promoter is not an adequate promoter for lentiviral vectors for induction of an immune response against an antigen. Moreover, the dectin-2 promoter is dendritic cell specific, which does not allow elimination of vectors that are integrated into other non-expressing cell types. Moreover, the dectin-2 promoter appears to contain an enhancer. Thus, the dectin-2 promoter is not a good promoter for lentiviral vectors for safety reasons.
Preferably, in immunotherapy, lentiviral vectors provide effective expression of the transgene that elicits a desired specific immune response. This requires that the expression is at a high level in APCs, such as dendritic cells.
It is also preferable that the cells transduced by the lentiviral vectors are eliminated by the immune response to provide a higher degree of safety. That is, the immune response generated against the transgene can elicit an immune response in the host sufficient to eliminate the cells that are transduced by the lentiviral vectors. The elimination of transduced cells eliminates the persistence of the lentiviral vector in the host, and possible secondary effects of the vector. In order for the transduced cells to be eliminated, expression is required in non-dendritic cells at a level that allows elimination by the immune response.
At the same time, the promoter should maximize immune stimulation through the key cells (i.e., dendritic cells) involved in the activation of naïve and memory T cells, and should minimize the risk of insertional mutagenesis and genotoxicity in stem cells, leading to malignancies. Thus, the promoter should have sufficiently high activity in dendritic and other cells, but not contain an enhancer. Based on these criteria, viral promoters, such as the CMV promoter, are not ideal because of the presence of strong enhancers. These criteria are summarized as follows:                1. high expression in antigen presenting cells, including dendritic cells, to induce maximal immune responses;        2. expression in other transduced cell types sufficient for elimination by the induced immune response; and        3. lack of an enhancer element to avoid insertional effects.        
Thus, a need exists in the art for improved vectors. The present invention fulfils these needs in the art.